The productivity of plants is limited by the three primary nutrients: nitrogen, phosphorous and potassium, in most natural and agricultural ecosystems. Generally nitrogen is the most important of the three limiting nutrients and the major components in fertilizers. Since nitrogen is usually the rate-limiting element in plant growth, most field crops have a fundamental dependence on inorganic nitrogenous fertilizer. The nitrogen source in fertilizer is usually ammonium nitrate, potassium nitrate, or urea (McAllister et al., 2012).
Increased nitrogen use efficiency by plants has a number of beneficial effects, for example, increased growth and yield when compared to conventional plants grown in nitrogen poor soils, and reduced requirement for the addition of nitrogenous fertilizers to crops (Good and Beatty, 2011a). Fertilizers account for a significant percentage of the costs associated with crop production, therefore using less fertilizer would reduce the producers' costs. A reduction in fertilizer application would also lessen the environmental damage resulting from extensive nitrogenous fertilizer use. Excess fertilizer application causes increased eutrophication, acid rain, soil acidification and the greenhouse effect. These environmental disasters cause further problems such as fish kills, loss of biodiversity, increased algal blooms, loss of arable land and accelerated global climate change, affecting the world population on both social and economic scales (Good and Beatty, 2011b).
Of the commercially grown plants, monocots (which include the main cereal crops) represent a large percentage of the crops grown in the world with approximately 217 million hectares of wheat and 158 million hectares of both maize and rice planted in 2007. Approximately half of the global calorie and protein requirement is derived from wheat, rice and maize. Rice is routinely used as a model crop for genetic and physiological studies in other monocot crops including maize, wheat, sugarcane, barley, sorghum, rye and grass. Rice has a small, diploid genome that is well conserved and syntenic across monocots (McAllister et al., 2013).
In the case of NUE plant engineering, a number of different genes have been evaluated for their role in increasing the efficacy of N uptake, utilization or remobilization in the plant. One way to improve the nitrogen use efficiency (NUE) of the cereal crops would be to improve the different components of NUE. NUE can be partitioned into N uptake efficiency (NUpE) and N utilization efficiency (NUtE; Good et al., 2004). NUtE can be further reduced to N assimilation and N remobilization. Therefore, increasing the efficiency of either N uptake or N utilization, could lead to an increase in NUE of the crop. There have been a number of single genes targeted as candidates for genetic engineering to try and increase the NUE of crop plants (reviewed in Good and Beatty, 2011a and McAllister et al., 2012). Many of these candidate genes are primary N uptake and assimilation genes such as nitrate and ammonia transporters, nitrate reductase, GS and GOGAT. Gene targets that have shown an NUE phenotype in a crop plant after bioengineering overexpression include genes that are not primary N assimilation genes, but instead are involved in N metabolism further downstream than GS (glutamine synthase) and GOGAT (Glutamine oxoglutarate aminotransferase) such as alanine aminotransferase (AlaAT), or are transcriptional regulators, such as Dof1 (Good et al., 2007; Shrawat et al., 2008; Yanagisawa et al., 2000; Yanigisawa et al., 2004). While some of these genes have shown some efficacy, in most cases the over-expression of these genes, under the specific promoter used, did not result in any significant increase in NUE or components of NUE (reviewed in Good and Beatty, 2011a and McAllister et al., 2012).